Offset printing is based on a printing scheme of first transferring a printing paste from a printing plate to a rubber blanket, and then printing the substrate with the transferred printing paste instead of directly transferring the paste to a substrate. Offset printing may be generally divided into flat type offset printing, in which a printing substrate is installed on a flat plate, and rotary press type offset printing, in which the printing substrate is installed on a cylindrical body. Of the two methods, rotary press offset printing is in general use. The rotary press printing may include a sheet-fed type printing process performing printing on each sheet, and a roll type printing process using a roll of printing paper. A printing unit of the rotary press printing generally includes a cliché, a blanket cylinder (rubber blanket), a pressure drum, an inking device supplying ink to a plate surface and a dampening water supply device supplying water. In the case of offset printing, a picture line is vivid and is printable, even on a substrate that is difficult to directly print to, and a printing method is simplified, while printing costs are cheap and thus usable in various fields. For example, offset printing is usable for electrode pattern formation, a fluorescent pattern formation for a plasma display panel, a color pattern formation for a color filter of a liquid crystal display, or the like.
In the offset printing method, dozens of microns of fine patterns may be formed by taking paste from the cliché through a blanket manufactured of silicon rubber for printing, and then setting the paste to the substrate. Meanwhile, the paste is applied to the cliché to be tens of microns (μm) thick using a doctor blade, and thereafter, a solvent in the paste which is offset printing ink, is dried in air or the solvent in the paste is absorbed into a silicon resin after transferring the paste from the cliché to the silicon blanket. Therefore, when the drying properties of the solvent of the paste in the air are too rapid, transcription properties of the paste from the cliché to the silicon rubber at the time of continuous printing may be deteriorated. In addition, when a swelling property of the silicon rubber due to the solvent is relatively great, the paste is dried in the silicon blanket such that a setting property on the substrate may be degraded, whereby printing quality may be deteriorated.
As such, printing properties, particularly, in continuous printing, depend upon air drying property of solvent and blanket swelling properties due to a solvent among several components constituting an offset printing paste. Moreover, as a printed area is increased such as in a plasma display panel (PDP) electrode printed pattern, a correlation between drying properties relating to the air drying speed of a solvent, and blanket swelling properties due to a solvent and printing quality become more important.
In general, an air drying speed of a solvent has a correlation with a boiling point of the solvent. In offset printing using a printing paste, a terpineol or butyl carbitol acetate (BCA) solvent having a boiling point of 200° C. or more in view of air drying properties of the solvent may be used. In the meantime, blanket swelling properties of the BCA and terpineol are 5 or less, respectively. However, the BCA or terpineol solvent has limitation in printing on an effective area in the case of continuous printing for performing hundreds of printings for a large area of an electrode. That is, an electrode pattern may be deteriorated by the accumulated blanket swelling due to the solvent. In addition, paste is stacked on a cliché due to the relatively rapid air drying properties of the solvent, thus reducing the height of a printed pattern. This phenomenon may be serious, as an interval between electrodes is smaller, and also, electrode pattern linearity may be degraded while causing defects in printing.
Therefore, printing technology able to provide excellent printing characteristics, in detail, excellent linearity, clarity and constant line height and line width, and the like, in the printed pattern, even in the case of multiple continuous printing processes, is required.